IWI insulates solid walls from the inside — ideal when external insulation isn’t possible or you want to phase the work room by room. Done right, it transforms cold rooms. Done wrong, it traps moisture. Here’s the full picture.
Choose IWI when: the façade can’t change (conservation areas, listed settings, attractive brickwork), budget is tighter, you want to phase work one room at a time, or only certain elevations are cold. Street-facing IWI plus rear-elevation EWI is a common hybrid on period terraces.
Expect roughly £60–£110 per m² installed — typically £600–£1,300 per average room including boarding and skim, before decoration. Performance scales with thickness: 60–100mm insulated plasterboard is common, costing 6–10cm of room dimension per treated wall. Only external-facing walls need treating — internal partitions don’t.
Solid walls dry to the inside; insulating internally changes that. A safe system needs a vapour-control strategy matched to the wall build-up, sealed service penetrations, careful junctions at joist ends (timber in cold walls is the decay risk), and returns of insulation along party walls and window reveals to kill cold bridging. PAS 2030/2035 installation gives you that design discipline — insist on it.
Internal wall insulation is an ECO4 measure for eligible households until 31 December 2026, and insulation features in England’s Warm Homes: Local Grant for EPC D–G homes meeting income criteria. Privately, IWI materials and installation carry 0% VAT.
Facts checked 21 August 2026. We link to official sources and show when facts were last verified. If a scheme changes, we update the page.
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